(NH4)(2)(VO)(VP)O-7, a layered structure comprising tetrahedral VPO7 groups

The new mixed-valence ammonium vanadophosphate (NH4)(2)(VO)(VP)O-7 (diammonium phosphorus divanadium octaoxide) is isostructural with K2VO(X(2)O(7)) (X = P, V). Its structure consists of [V2PO8](infinity) layers of corner-sharing VO5 pyramids and VPO7 groups, interleaved with NH4+ ions. The unusual features of this structure are the existence of the VPO7 groups involving VO4 and PO4 tetrahedra, and the simultaneous presence of (VO5)-O-IV and (VO4)-O-V polyhedra, observed here for the first time in this class of compounds. The VPO7 groups are statistically distributed in the framework. Magnetic measurements and bond-valence calculations confirm the mixed-valence character of this phase and show that the VO5 pyramids and VO4 tetrahedra are occupied by V-IV and V-V, respectively.